Introduction

The Shell is a hard casing that provides a friendly environment to the user, while protecting each user from every other one. When a user logs into a Unix system, the operating system automatically starts a unique copy of the Shell under which the user can perform any of the functions available.  The original Unix shell was rewritten by S.R.Bourne in 1975, giving us the current version of the shell known as the Bourne Shell. Students and professors at the Berkeley campus created another version of the Shell, known as the C Shell, that is useful for C language programmers. Later another shell  called the T shell (tcsh)  was developed,  which is an enhanced but completely compatible  version  of the  Berkeley  Unix C shell. There are some new features in the T shell which are not present in the C shell. A shell is basically a command line interpretor. It takes commands and executes them. As such, it implements a programming language, called the Shell Script Language. Programs that are interpreted/executed by the shell are called Shell Scripts.

There are several on-line tutorials on shell script languages.

 For C Shell :
 http://www.eng.hawaii.edu/Tutor/csh.html

For T Shell:

manual pages for the T shell

For Bourne Shell:
 http://207.213.123.70/book
http://unixhelp.ed.ac.uk/scrpt/scrpt2.html
 http://theory.uwinnipeg.ca/UNIXhelp/shell/shell_help.html

The Linux machines at NMSU CS Department have both the Bourne shell and the T shell. So, users can program in any one of the shells.

You can check to see what shell you are running by typing env | grep SHELL at the command line, and by looking at the very last name in the path given.
The output will look something like:

anson:[13] env | grep SHELL
SHELL=/local/common/bin/tcsh
anson:[14]

which shows that the shell that I am running is T Shell.
 

How to run Shell Script

Since shell script is basically a command line interpreter, it is easier to write several commands in a file and make the file executable to execute them simultaneously, instead of writing them one at a time on the command line to execute them.  First of all you have to prepare a text file that contains the shell script. If you choose to run the script as an executable file, you must add the following line to the very beginning of your script file:

#!/bin/sh
if you want to run it in Bourne shell, or

#!/bin/tcsh or
#!/bin/csh
if you want to run it in Tshell (which is very similar to the C shell) or C shell. In the NMSU CS Department environment, both tcsh and csh  will execute the script in T shell.

 Note: If the above line doesn't start from the first column of the line, then the line doesn't get executed and the script is considered to be the default shell, in this case T shell script.

Then set execute permissions to your script file using the following command:

chmod  u+x  my_file

Then call your script file directly in the command line as:

 my_file

This is the sequence in which you should type in the commands:

anson:[11] chmod u+x my_file
anson:[12] my_file
    intro.ps
    my_file
    test

anson:[13]

The program my_file outputs the name of all the files my directory that has been modified/written in the month of September:

#!/bin/sh
for file in *
        do
                mon=`ls -l $file|cut -c43-45`
                if [ "$mon" = "Sep" ]
                        then
                            echo $file
                fi
        done
 
 

Bourne shell
 

• First of all include the following  at the top of your shell script from the first position.

• The assignment statement :

          max=0
           this assigns 0 to the variable max. Notice that no spaces are allowed around the equality symbol.

          max=$linecount
            this assigns the value stored in the variable linecount to the variable max. In order to access the value of a variable , you precede the variable name with a dollar sign($).
        Notice that both linecount and max are variables, but since 0 or the value of linecount is getting assigned to the variable max, the '$' sign is not added to its beginning, otherwise all variables are identified by the '$' sign before them.
        Also, note that each of the assignment operators should be written in a seperate line.

• The conditional statement:

        if [ test condition ]
            then
                 process 1
        else
                 process 2
            fi

                where the test conditions are the conditions you are checking for before deciding whether you have to execute process 1 or process 2. If you want the default processing path to do nothing, you can just write the if condition without the else part as:

                 if [ test condition ]
           then
               process 1
       fi

           Note that the test conditions are conditions constructed from variables, operators and constants and they should be enclosed in square brackets. The test conditions can contain operators. Some of the Arithmetic Test Operators are as follows:
                -lt : less than
                -le : less than or equal
                -gt : greater than
                -ge : greater than or equal
                -ne : not equal
                -eq : equal

      For example:

                if [ $linecount -gt $max ]
                    then
                           echo "the end"
                else
                           echo $linecount
                fi

•  The for control structure permits looping through a series of actions while changing a variable name specified on the for command line. The common form of the for structure is as follows:

    Note that the '$' is not used on the loop variable, but it is used on the variable inside the loop.
         An example of the for loop is given below:
           for file in *
         do
               echo $file
         done

    This prints out the filenames of all the files that exist in the current directory (the '*' refers to current directory). The output after executing the above for loop is as follows:

        singapore:[20] my_forloop
        intro.ps
        my_file
        test
        my_forloop

     singapore:[21]

•  To print something on the screen, use the following command:

      For example:

       echo $max

      This prints the value of the variable max on the screen.
 

•     To add comments in your file you have to use the pound symbol (#) at the beginning of the line to be commented out. Any line that begins with the pound symbol is taken as a commented line.

• The wc command can be used to count the number of lines in a file and also to get the corresponding filename.  In order to assign a value returned by a command to the variable use a command of the form:

            Notice the usage of back quotes around the unix command. The back quotation marks(`) are used when you want to use the results of a command in another command. . For example, if you want  to set the value of the variable contents equal to the list of files in the current directory, type the followingcommand:
contents=`ls`

            Here is an usage of the wc command on the command line itself:

      anson:[15] wc -l my_file
              11 my_file
     anson:[16]

            where the first argument is the number of lines in the file, and the second is the name of the file; i.e. the file my_file consists of 11 lines.

            In order to seperate out the number of lines from the wc command output, you can use awk in conjunction with wc.

            linecount=`wc -l my_file | awk '{print $1}'`

      This assigns to the variable linecount, the number of lines in the filename my_file.
  Notice that the awk command parameter is in regular quotes, while the unix commands are in back quotes.
 
 

T shell

        There are some differences in the syntax between the T shell script and the Bourne shell script .

• Include the following  at the top of your shell script:

• The assignment statement :

            set max=0

             this assigns 0 to the variable max. Notice that no spaces are allowed around the equality symbol.

            set max=$linecount

            this assigns the value stored in the variable linecount to the variable max.
        Notice that both linecount and max are variables, but since 0 or the value of linecount is getting assigned to the variable max, the '$' sign is not added to its beginning, otherwise all variables are identified by the '$' sign before them.
        Also, note that each of the assignment operators should be written in a seperate line.
 

• The conditional statement:

                 if ( condition ) then
                     statements
                endif
         The typical if-then-elsestatement looks like this:

        if ( test condition )
            then
                 process 1
        else
                 process 2
        endif

                where the test conditions are the conditions you are checking for before deciding whether you have to execute process 1 or process 2. If you want the default processing path to do nothing, you can just write the if condition without the else part as:

                 if ( test condition )
           then
               process 1
       endif
       Note that the test conditions are conditions constructed from variables, operators and constants and they should be enclosed in ordinary round brackets. The test conditions can contain operators. Some of the Arithmetic Test Operators are as follows:

                <  : less than
                <= : less than or equal
                >  : greater than
                >= : greater than or equal
                != : not equal
                == : equal

       For example:

                if ( $linecount > $max ) then
                      echo "the end"
                else
                           echo $linecount
                endif

 
• The for structure equivalent of the Borne shell is the foreach structure in the T Shell.. The foreach control structure permits looping through a series of actions while changing a variable name specified on the foreach command line. The common form of the foreach  structure is as follows:

           foreach variable ( value1 )

           repeat action on $variable
        end

    Note that the '$' is not used on the loop variable, but it is used on the variable inside the loop.
         An example of the for loop is given below:
           foreach file (*.c)

               echo $file
      end

    This prints out the filenames of all files in the current directory with *.c extension. The output after executing the above foreach  loop is as follows:

            singapore:[20] my_foreachloop
        test1.c
        test2.c

     singapore:[21]

•  To print something on the screen, use the following command:

      For example

                   echo $max $name

       This prints the value of the variable max and name on the screen. Note that if the echo command is the last command in the script file, sometimes  it doesn't print on the standard output because of buffering. To fix it, put an additional echo command at the end of the script file.
 

•     To add comments in your file you have to use the pound symbol (#) at the beginning of the line to be commented out. Any line that begins with the pound symbol is taken as a commented line.

• The wc command can be used to count the number of lines in a file and also to get the corresponding filename.  In order to assign a value returned by a command to the variable use a command of the form:

            Notice the usage of back quotes around the unix command.

            Here is an usage of the wc command on the command line itself:

      anson:[15] wc -l my_file
              11 my_file
     anson:[16]

            where the first argument is the number of lines in the file, and the second is the name of the file; i.e. the file my_file consists of 11 lines.

            In order to seperate out the number of lines from the wc command output, you can use awk in conjunction with wc.

            linecount=`wc -l my_file | awk '{print $1}'`

      This assigns to the variable linecount, the number of lines in the filename my_file.

           Here is an usage of wc in conjunction with awk on the command line itself:

        anson:[17] wc -l my_file | awk '{print $1}'
            11
   anson:[18]

           Notice that the awk command parameter is in regular quotes, while the unix commands are in back quotes.
 
 

 Homework Assignment (10 points)

Write a t shell or Bourne shell script to find out the C/C++ header file  from all header files (*.h) in /usr/include ( by the way, this is the default include library for the gcc compiler. If you have any problems with include files, you can check them out in this library) with the biggest and the smallest number of lines.

Print out the name , date and number of lines of the max and min files.  If there are several files with the same maximum/minimum number of lines, print out the name of any one of them.  Also, provide a brief explanation of what your script  does by including some comments in your script file.
 

 This homework is to be e-mailed to the TA before midnight on the due date. Name of the program should be your login Name.